Multiturn absolute encoder

ABSTRACT

Angle transducer unit of the multiturn measuring type for absolute measurement of the angular position of a rotatable shaft. A first transducer (10) of the absolute angular measurement type with a drive shaft (3) is non-rotatably mounted on the rotatable shaft. A second angle transducer (11) of the absolute angular measurement type has a driven shaft (4). There are gear wheels (1,2) mounted on the drive and driven shafts, these gear wheels being mutually in mesh to form a gear for up or down gearing of the rotational rate of the drive shaft. The two angle transducers, e.g. resolvers, are intended for being conventionally connected to an apparatus for measuring the differential angular position of the shafts. Novel to the angle transducer unit is that the gear wheels include an internally toothed gear wheel (2) and an externally toothed gear wheel (1). The latter rolls on the internally toothed gear wheel (2) and has a number of teeth which is only some few teeth, preferably only one tooth less than the number of teeth of the internally toothed gear wheel, and is mounted on one of the shafts 3 and 4. The internally toothed gear wheel (2) is mounted on the other shaft. Cylindrical housing parts are disposed for mounting the shafts mutually eccentric.

The present invention relates to angle transducers of the multiturntype, where the absolute angular position of a shaft which can rotate ismeasured, i.e. the total number of revolutions plus an angle less than360° is measured from a fixed initial, reference position of the shaft.

For reasons of safety, angle transducers of the kind mentioned above arecoming into greater and greater use with industrial robots. The reasonsfor this is that should a current failure occur it is essential to keeptrack of the absolute angular positions of the respective rotatingshafts in the robot, because when power returns the robot must continueits motion from where it left off when current failed. In other words, acurrent failure must not mean that the respective angle transducersrecord zero values, since this would mean that when current returns therespective shafts in it would start from their respective zero angularpositions, which could give the robot a completely incorrect motion andcould mean danger to life for persons in the vicinity of the robot. Suchundesired zero setting of angular position occurs if the respectiveangle transducer does not measure the shaft angle absolutely but onlyrelative to the angle at which the shaft happens to stop.

Angle transducers of the kind mentioned are known, and usually comprisetwo transducers, e.g. resolvers or electro-optical transducers arrangedside by side, and mutually connected via a spur gear. One of thetransducers is connected to a rotating shaft of the robot. The gearcomprises two gear wheels, one of which has 64 teeth, for example, andthe other 65 teeth. The differential rotation of the resolvers enablesabsolute measurement of the total angle passed through by the rotatingshaft. Any angular position of the two resolvers corresponds to anunambiguously determined total turning angle of the rotating shaft. Thedifferential turning angle is determined by conventional electricalcircuits, which may be of the phase-sensitive type or the amplitudesensitive type.

The angle transducer unit described in the last paragraph has pluralityof disadvantages. The transducers in it are situated side by side andare mutually connected to the gear, resulting in that the cylindricalhousing into which the whole transducer unit is usually built has alarge diameter. In certain applications, the space taken up by thetransducer housing on the machine detail where the rotating shaft issituated is disadvantageously large. The gear, which solely comprisestwo externally toothed spur gears in mesh, is not free from backlash. Toprevent this backlash, one of the gear wheels is provided with aspringbiased gear wheel which urges the gear wheels in the gear intomesh with each other during all rotational phases of the shaft. Theteeth of the gear are subjected to large loads due to the large momentof inertia of the driven gear wheel and the high momentary accelerationsoccuring for rapid positional changes, or when the rotating shaftchanges its direction of rotation. Such large accelerations cause wearon the teeth after a while. Only one tooth on one gear wheel issimultaneously in engagement with a tooth on the other gear wheel of thegear, resulting in that only these two teeth take up the accelerationstresses. This causes wear on the teeth of the gear. The meshing factorof a conventional spur gear is normally about 1.6-2. Both gear wheels ina conventional spur gear rotate at high absolute speed and the mutual,relative tooth speed is very high. The high relative tooth speed causeswear on the teeth. All this reduces the life of the known angletransducer. Due to the high tooth speeds and thereby the highcentrifugal force, the gear lubricant will be urged away from the teeth.To obtain good, durable lubrication it is necessary for the gear wheelsto be disposed in an oil bath.

The present invention has the object of providing an angle transducer ofthe kind mentioned in the introduction, which, while avoiding the abovementioned disadvantages has long life, and for this purpose has a gearwith small moment of inertia, low relative tooth speed and a highmeshing factor as well as the least possible backlash without usingspring biased bear wheels.

The characterizing features of the invention are disclosed in theaccompanying claims.

From a study of the claims it will be understood that since theexternally toothed gear wheel only has few teeth, preferably only onetooth, less than the internally toothed gear wheel there is obtained:

a) in the case where the internally toothed gear wheel is mounted on theshaft of the driving transducer and the externally toothed gear wheel onthe shaft of the driven transducer that when the driving shaft hasrotated one revolution, the driven shaft has rotated one revolution plusthe angle taken up by the difference in the number of teeth, a smallupward gearing thus being obtained,

b) in the case where the externally toothed gear wheel is mounted on thedriving shaft and the internally toothed gear wheel on the driven shaft,that when the rotating shaft has rotated one revolution the driven shafthas rotated one revolution minus the angle occupied by the difference inthe number of teeth.

If the driving rotational rate is high, both gear wheels do indeedrotate at high rotational rates, but the relative tooth speed of thegear wheels is very low, more specifically the rotational rate of thedriving shaft divided by the number of teeth on the gear wheel mountedon the driving shaft. Due to this low relative tooth spaced, the wear onthe teeth is reduced considerably, compared with the gear conventionallyused.

As the externally toothed gear rolls against the internally toothedgear, there are a plurality of teeth in mesh simultaneously, i.e. thegear has a greater meshing factor, which is about 8. In the inventivegear a large number of teeth will thus take up the acceleration stressesin the meshing teeth. By distributing the load caused by theacceleration over so many teeth, the life of the teeth, and thereby thatof the gear is increased.

By turning the houses relative each other, the backlash can be broughtto a least possible value, which is advantageous from the manufacturingaspect, since the teeth do not need to be mashined with particularlytight tolerances. The situation is actually the reverse, i.e. ratherlarge tolerances can be permitted and these can be compensated byturning the houses relative each other so that the externally toothedgear wheel comes into a desired degree of engagement with the internallytoothed gear wheel. There is thus avoided the use of separate, springbiased gear wheels.

Due to the end-on-end mounting of the angle transducers, the amount ofspace taken up by the transducer units on the machine detail where theoutput shaft is situated is reduced. In other words, the inventiveapparatus requires extension in length rather that in width.

The invention will now be described in more detail hereinafter inconnection with the accompanying drawings, where

FIG. 1 is an exploded view showing the apparatus in accordance with theinvention,

FIG. 2 is a section of one angle transducer mounted in its housing,

FIG. 3 is a section of the other transducer mounted in its housing,

FIG. 4 is a section showing the transducers according to FIG. 2 and 3fitted together into a unit,

FIG. 5 is a section along the line V--V in FIG. 4, the eccentricitiesbeing heavily exaggerated,

FIG. 6 is a view showing the transducers fitted together in analternative embodiment, and

FIG. 7 is a view showing the transducers fitted together in yet anotheralternate embodiment.

It will be seen from FIG. 1 that the apparatus in accordance with thepresent invention includes an externally toothed gear wheel 1, aninternally toothed gear wheel 2, a drive shaft 3, a driven shaft 4, afirst housing 5, a second housing 6 and two cover plates 7 and 8.

The externally toothed gear wheel 1 has 63 teeth and the internallytoothed gear wheel 2 has 64 teeth 9 against which the externally toothedgear wheel 1 is to mesh.

The gear wheels are fabricated in the manner described in my Swedishpatent application 87 00291-1. The units 1-6 form a gear of a naturesuch that when the gear drive shaft 3 has rotated 64 revolutions itsdriven shaft 4 has rotated 63 revolutions. The drive shaft 3 of the gearis a part of an angle transducer 10 of conventional kind, e.g. aresolver with a cylindrical housing. The shaft 3 is preferably straightthrough and the part of its projecting out to the left from the housingin FIG. 1 is intended to be non-rotatably connected to the unillustratedrotating shaft which is to have its absolute angular positiondetermined. In the same way, the driven shaft 4 of the gear is a shaftin an angle transducer 11, e.g. a resolver. This transducer 11 also hasa cylindrical housing and is intended for fitting into the other housing6.

Booth housings 5 and 6 are in the form of cylindrical sleeves. In thefirst cylindrical housing 5 there is a cylindrical shoulder 12, definingthe depth to which the first transducer 10 can be thrust into the firsthousing 5. In the second housing 6, there is correspondingly acylindrical shoulder 13 defining the depth to which the cylindricaltransducer 11 can be thrust. Both transducers are fixed non-rotatably inthe respective housing 5, 6 e.g. by glue or cement. FIG. 2 illustratesthe step in assembly where the transducer 10 is fitted into the housing5, the cover plate 7 placed over the drive shaft of the gear, subsequentto which the externally toothed gear wheel is non-rotatably attached,e.g. by glue, on the shaft 3. FIG. 3 illustrates how the transducer 11is fitted into the other housing 6, after which the cover plate 10 isplaced over the driven shaft 4 of the gear and finally the internallytoothed gear wheel 2 is fixed, e.g. by glue, on the shaft 4. The coverplates 7 and 8 serve as barriers to the penetration of glue along therespective shafts 3, 4. The gear is lubricated.

The shafts 3 and 4 are parallel with their centre lines 15a and 15b atmutual spacing e, see FIG. 5. This spacing e enables the externallytoothed gear wheel 1 to roll round the internally toothed gear wheel 2,and can thus be regarded as the measure of an eccentric mounting of thedriven shaft 3, where e is the difference between the pitch radii of thegear wheels. As will be seen from FIG. 2, there is a cylindrical bore14a in the housing 5. As will be seen from FIG. 5, this bore 14a iseccentric relative the longitudinal symmetry axis 15a of the housing.The eccentricity is denoted by f. As will be seen form FIG. 3, there isa cylindrically machined surface 14b on one end portion of the housing6. This surface 14b is similarly arranged eccentrically relative thelongitudinal symmetry axis 15b of this housing 6, as will be seen fromFIG. 5. The eccentricity is the vectorial sum (magnitude and direction)of e and f relative the longitudinal symmetry axis 15b.

The surface 14b is intended to be a light drive fit in the bore 14a, aswill be seen from FIG. 4, which illustrates the two transducer parts 10(FIG. 2 and 11) in FIG. 3 assembled together such that the gear wheelsare in mesh and a closed space 16 has been formed, which is axiallydefined partly by the cover plate 7 and partly by the gear wheel 2. Thecover plate 7 thus prevents departure of the lubricant from the space 16and thus has the double function of preventing penetration of glue anddeparture of lubricant from the space 16 as a result of the action ofcentrifugal force. In turn, this considerably extends the time intervalsat which the transducer must be lubricated. The cover plate 7 shouldhave a radial extension such that it covers the end wall 17 of theinternally toothed gear wheel 2. Due to the low relative rotational rateof the two gear wheels 1 and 2, the cover plate 7 is only subjected tominimum wear.

By turning the housing 5 and 6 relative to each other, the gear wheelsare displaced in mutual relationship, due to the eccentrically arrangedfit 14a, 14b, and in this way the backlash between the two gear wheelsis reduced to a minimum. After adjusting this relative angularrelationship, the housings 5, 6 are fixed to each other, e.g. with theaid of glue.

The embodiment of the invention described hereinbefore can be modifiedin many ways and varied within the scope of the inventive concept.Instead of the eccentric fit 14a, 14b, which has, inter alia, thefunction of allowing radial displacement of the gear wheels 1 and 2relative each other, a linear displacement of the gear wheels can becarried out with the aid of other means.

Glueing, or cementing has been proposed above as means for fixing thegear wheels to the shafts, for fixing the transducers in theirrespective housings and for fixing the housings relative each other, butit will be understood that other means, e.g. set screws, can be used.The number of teeth has been stated to be 63 and 64 respectively, but itwill be understood that other values can be used, as well as having adifference in the respective number of teeth other than 1, e.g. 2, 3 ormore.

The housing parts 5, 6 have been shown above as separate units, but itwill be understood that a housing 5' can be integral with the housing ofthe transducer 10', and a housing 6' can be integral with the housing ofthe transducer 11' as depicted in FIG. 6.

In addition, several apparatuses in accordance with the invention can beconnected in series for increasing the measuring range. For example, asdepicted in FIG. 7, an extra angle transducer 11" can be provided. Angletransducer 11" includes a gear 2", similar to gear 2, affixed to adriven shaft 4" similar to driven shaft 4. In addition, angle transducer11 includes a gear 1", similar to gear 1, affixed to a drive shaft 3"similar to drive shaft 3. The additional angle transducer 11" is coupledto angle transducer 11 through an eccentric means of the type alreadydescribed herein including sleeves 5", 6" which are similar to sleeves5, 6.

I claim:
 1. Angle transducer unit of the multiturn type for absolutemeasurement of the angular position of a rotatable shaft, including afirst angle transducer of the absolute angular measurement type, with adrive shaft, which is nonrotatably mounted on the rotatable shaft, saidfirst angle transducer including a first housing, a second angletransducer of the absolute angular measurement type, with a drivenshaft, said second angle transducer including a second housing, and gearwheels respectively mounted on the drive and driven shafts, said gearwheels being in mesh with each other to form a gear for up or downgearing of the drive shaft rotational rate, both angle transducers beingintended for convention connection to an apparatus for measuring thedifferential angular position of the drive and driven shaftscharacterized in that said gear wheels are respectively an internallytoothed gear wheel and an externally toothed gear wheel, the latter(a)rolling against the internally toothed gear wheel, (b) having a numberof teeth which is only some few teeth less than the number of teeth onthe internally toothed gear wheel, and (c) being mounted on one of saiddrive or driven shaft, the internally toothed gear wheel being mountedon the other of said drive or driven shaft, and in that eccentric meansis provided in the form of a bore provided in said first housing and acorresponding cylindrical surface provided on said second housing, saidfirst housing being attached to said second housing such that saidcylindrical surface is fitted into said bore for permitting the assemblyof the shafts eccentrically in relation to each other.
 2. Angletransducer unit as claimed in claim 1, characterized in that theinternally toothed gear wheel is mounted on the drive shaft and that theexternally toothed gear wheel is mounted on the driven shaft, which thenrotates at a rate which is somewhat higher than that for the driveshaft.
 3. Angle transducer unit as claimed in claim 1, characterized inthat the externally toothed gear wheel is mounted on the drive shaft andthat the internally toothed gear wheel is mounted on the driven shaft,which then rotates at a rate which is somewhat less than for the driveshaft.
 4. Angle transducer unit as claimed in claim 1, wherein saidfirst housing is a first cylindrical sleeve one end of which includessaid bore oriented eccentrically relative the longitudinal symmetry axisof said first sleeve, and wherein said second housing is a secondcylindrical sleeve one end of which includes said correspondingcylindrical surface, said cylindrical surface being orientedeccentrically relative the longitudinal axis of said second sleeve toform a projecting neck intended to be a light drive fit in said bore,the centers of said bore and said cylindrical surface being mutuallycoinciding and eccentric to the longitudinal symmetry axis of said firstsleeve by an amount (f) and also eccentric to the longitudinal symmetryaxis of said second sleeve by the vectorial sum of e and f where e isthe difference between the pitch radii of the gear wheels and f is asuitable value for reducing the backlash between the gear wheels byturning the first and second sleeves relative each other.
 5. Angletransducer unit as claimed in claim 4, characterized in that said firstsleeve is affixed to said first angle transducer and said second sleeveis affixed to said second angle transducer.
 6. Angle transducer unit asclaimed in claim 1 characterized in that said first housing is integralwith said first angle transducer and said second housing is integralwith said second angle transducer.
 7. Angle transducer unit as claimedin claim 1, characterized in that said first and second housings areglued to each other.
 8. Angle transducer unit as claimed in claim 1,characterized in that on the driven shaft there are mounted an extragear, extra angle transducer and extra eccentric means for increasingthe angle measurement range.
 9. Angle transducer unit as claimed inclaim 8, characterized by a sealing cover plate being fixedly arrangedon the drive shaft and extending over the side face of the internallytoothed gear wheel for sealing it.
 10. Angle transducer unit as claimedin claim 2, wherein said first housing is a first cylindrical sleeve oneend of which includes said bore oriented eccentrically relative thelongitudinal symmetry axis of said first sleeve, and wherein said secondhousing is a second cylindrical sleeve one end of which includes saidcorresponding cylindrical surface, said cylindrical surface beingoriented eccentrically relative the longitudinal axis of said secondsleeve to form a projecting neck intended to be a light drive fit insaid bore, the centers of said bore and said cylindrical surface beingmutually coinciding and eccentric to the longitudinal symmetry axis ofsaid first sleeve by an amount (f) and also eccentric to thelongitudinal symmetry axis of said second sleeve by the vectorial sum ofe and f where e is the difference between the pitch radii of the gearwheels and f is a suitable value for reducing the backlash between thegear wheels by turning the first and second sleeves relative each other.11. Angle transducer unit as claimed in claim 3, wherein said firsthousing is a first cylindrical sleeve one end of which includes saidbore oriented eccentrically relative the longitudinal symmetry axis ofsaid first sleeve, and wherein said second housing is a secondcylindrical sleeve one end of which includes said correspondingcylindrical surface, said cylindrical surface being orientedeccentrically relative the longitudinal axis of said second sleeve toform a projecting neck intended to be a light drive fit in said bore,the centers of said bore and said cylindrical surface being mutuallycoinciding and eccentric to the longitudinal symmetry axis of said firstsleeve by an amount (f) and also eccentric to the longitudinal symmetryaxis of said second sleeve by the vectorial sum of e and f where e isthe difference between the pitch radii of the gear wheels and f is asuitable value for reducing the backlash between the gear wheels byturning the first and second sleeves relative each other.
 12. Angletransducer unit as claimed in claim 4 characterized in that said firsthousing is integral with said first angle transducer ad said secondhousing is integral with said second angle transducer.
 13. Angletransducer unit as claimed in claim 4, characterized in that said firstand second housings are glued to each other.
 14. Angle transducer unitas claimed in claim 3, characterized in that on the driven shaft thereare mounted an extra gear, extra angle transducer and extra eccentricmeans for increasing the angle measurement range.
 15. Angle transducerunit as claimed in claim 4, characterized in that on the driven shaftthere are mounted an extra gear, extra angle transducer and extraeccentric means for increasing the angle measurement range.
 16. Angletransducer unit as claimed in claim 5, characterized in that on thedriven shaft there are mounted an extra gear, extra angle transducer andextra eccentric means for increasing the angle measurement range. 17.Angle transducer unit as claimed in claim 4, characterized by a sealingcover plate being fixedly arranged on the drive shaft and extending overthe side face of the internally toothed gear wheel for sealing it. 18.Angle transducer unit as claimed in claim 14, characterized by a sealingcover plate being fixedly arranged on the drive shaft and extending overthe side face of the internally toothed gear wheel for sealing it. 19.Angle transducer unit as claimed in claim 15, characterized by a sealingcover plate being fixedly arranged on the drive shaft and extending overthe side face of the internally toothed gear wheel for sealing it. 20.Angle transducer unit as claimed in claim 16, characterized by a sealingcover plate being fixedly arranged on the drive shaft and extending overthe side face of the internally toothed gear wheel for sealing it.